Not so long ago, people thought corals were a kind of plant that grew in the sea. But in the 18th century a scientist used a microscope to look at corals. He realised that corals were actually a kind of animal. They have tiny tentacles and 'stomachs'. They can even catch and eat other small animals such as plankton. Plankton are very small organisms that float in the sea.

But corals live mostly on a small variety of algae, or seaweed. Like other plants, they live on sunlight. This is why corals are found in shallow waters in tropical places with lots of sunlight. The biggest collection of corals in the world is the Great Barrier Reef in northern Queensland, Australia.

Important ecosystems

Coral reefs form the largest living structures in the world. They are also important ecosystems. Coral reefs only cover about 1 per cent of the surface of the earth. But 25 per cent of all species we know live in coral ecosystems. About 500 million people depend the ecosystems for food in terms of fishing and so on. And because coral reefs form in shallow waters close to the coast, they protect shorelines from waves. This makes it possible for people to live close to the sea. Corals have even provided treatments for diseases.

Under threat

Australia's Great Barrier Reef is so big it can be seen from space. It's actually a system of reefs. It has more than 2,900 reefs in total. The best way to think of a coral reef is as a structure that forms from all the skeletons of dead corals. A reef is like a coral graveyard.

The Great Barrier Reef supports many forms of life. This is called an ecosystem. Whales, dolphins, turtles and even saltwater crocodiles live there. Sadly, it is threatened by global warming, overfishing and pollution. Fertilisers and pesticides from farms make their way into rivers that flow into the sea. They poison the corals. Corals are also being attacked by the crown-of-thorns starfish. Parts of the reef are also being choked by seaweed.